docs: Pass Docker credentials in a safer way

What does this MR do and why?

docs: Pass Docker credentials in a safer way.

Passing the password/token on the command line is insecure and can easily leak, e.g. via the ps command line tool. The here-string syntax <<<$var avoids this problem.

While there improved language that could be misread as CI_REGISTRY_PASSWORD being a short-lived token. According to https://docs.gitlab.com/ee/ci/jobs/ci_job_token.html#gitlab-cicd-job-token-security the token is long-lifed and needs to be kept secret.

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